1. Field of the Invention
This invention relates to certain novel cyanato group containing phenolic resins, also known as phenolic cyanate resin and to a process of preparing same. More particularly, this invention relates to such resins which have improved properties and to a process for preparing such resins.
2. Prior Art
Phenolic resins are a class of synthetic materials that have grown continuously in terms of volume and applications for over several decades. The building blocks used in greatest volume are phenol and formaldehyde. Other important phenolic starting materials are the alkyl-substituted phenols, including cresols, xylenols, p-tert-butyl-phenol, p-phenylphenol, and nonylphenol. Diphenols, e.g., resorcinol (1,3-benzenediol) and bisphenol-A [bis-A or 2,2-bis(4-hydroxylphenyl)propane], are employed in smaller quantities for applications requiring special properties. In addition to formaldehyde, acetaldehyde or furfuraldehyde sometimes are employed but in much smaller quantities. The greater latitude in molecular structure, which is provided by varying the raw materials, chemistry, and manufacturing process, has made possible an extremely large number of applications for these products as a result of the array of physical properties that arise from the synthetic options.
The early investigation of the reaction of phenol and formaldehyde began with the work of yon Baeyer and others in the early 1870's as an extension of phenol based dye chemistry. The initial experiments result in soluble, amorphous products whose properties elicited little interest. Insoluble, cross-linked products also were reported in the lates 1880's, but these products also were not perceived as useful materials. In 1888, the first patent for a phenolic-resin product intended for use as a hard-rubber substitute was granted. The first commercial product was introduced as a shellac substitute by the Louis Bluner Company in the early 1900's. Process patents were issued in 1894 and 1895 for ortho- and para-methylolphenol, respectively.
Key innovations in early phenolic-resin manufacture included control of the molecular structure and the use of heat and pressure to achieve desirable physical properties in filled compositions. Studies in the use of acidic or basic catalysts and of changes in the molar ratio of formaldehyde to phenol resulted in the definition of two classes of polymeric materials which are referred to as Bakelite resins. Caustic-catalyzed products, which are prepared with greater than a 1:1 mol ratio of formaldehyde to phenol, can be used to form cross-linked, insoluble, and infusible compositions in a controlled fashion. With less than a 1:1 mol ratio of formaldehyde to phenol, the resultant products remain soluble; furthermore, acid catalysis yields permanently stable compositions, whereas base-catalyzed materials can be advanced in molecular weight and viscosity. Possibly of greatest importance to early commercialization, however, was the reduction to practice of the use of heat and pressure to produce essentially void-free molding compositions.
Resole resins are made with an alkaline catalyst and a molar excess of formaldehyde. Novolak or novolac resins are prepared with an acid catalyst and less than one mol of formaldehyde per mol of phenol. The initial reaction involved in the preparation of resolated novolacs is carried out with an acid catalyst and less than a 1:1 mol ratio of formaldehyde to phenol. After formation of the novolac, the pH is adjusted so that the reaction mixture is basic and additional formaldehyde is added. Resoles and resolated novolaks are inherently thermosetting and require no curing agent for advancement. Novolacs, by comparison, are themoplastic and require the addition of a curing agent, the most common being either hexamethylene-tetramine or a resole. The stages of molecular weight advancement are characterized by liquid or solid phenolic polymer which is soluble in certain organic solvents and is fusible; solid resin which is insoluble but swelled by organic solvents and, although softened by heat, exhibits essentially no flow; and an insoluble, infusible product which is not swelled by solvents nor softened by heat, i.e., the system is in a highly cross-linked state.
Phenolic resins have many uses. For example, such materials are used as bonding agents in friction materials such as brake linings, clutch facings, transmission bonds and the like. For example, U.S. Pat. Nos. 4,268,157; 4,069,108; 4,268,657; 4,218,361; 4,219,452; and 3,966,670 describe various friction materials in which a phenolic resin is employed as the bonding agent. Phenolics are also used as molding materials, and as coatings and adhesives. Phenolic resins developed for non-flammability and long-term temperature stability to 230.degree. C. have been studied in carbon-fiber composites. Potential for such composites lies in advanced aircraft application.
While present day phenolics exhibit several beneficial properties, they suffer from a number of disadvantages which restrict their utility. For example, such materials exhibit less than desirable thermal oxidative stability. Other major problems of present day phenolic technology include a need for auxilary chemicals such as hexamethylenetetramine to crosslink the phenolic which often results in the production of volatile by-products such as ammonia during crosslinking is often extensive and is not controllable.
Various modifications to phenolics have been proposed to obviate certain of the disadvantages attendant to these resins. For example, epich orohydrin has been reacted with the hydroxyl groups of novolak forming epoxy novolak. Moreover, n-chloro-2-propene has been reacted with the hydroxyl groups of novolac to form the corresponding form methylon resin.
Japanese Patent Publications Nos. 59-149918, and 58-34822 describe a method of preparing a phenolic resin containing cyanate groups. In this method, a trialkyl ammonium salt of a phenol novolak is reacted with excess cyano halogen in an organic solvent such as methylene chloride. The ammonium by-product salt is separated from the reaction mixture by extraction with water. Several disadvantages are attendant to the process of these references. The reaction is limited to being conducted in a water immiscible solvent. As a result it suitable only for cyanation of low molecular weight novolac resin below 320 Mn. We have found that the method disclosed in these references results in a phenolic cyanate resin which release smoke (volatiles) during curing at 155.degree. C. or above.
U.S. Pat. No. 3,448,079 describes aromatic cyanic acid esters produced by the reaction of phenolic resins with cyanogen halide in which the hydroxyl groups of the phenol-formaldehyde resins are replaced with cyanic acid ester groups, and process for producing same. U.S. Pat. No. 3,444,137 describes curable phenol-aldehyde resins characterized by molecules which contain a cyano group, an amine nitrogen atom, a phenyl group and a substituted hydroxyl group, such molecules having been made by reacting a phenol, formaldehyde and a cyano substituted primary or secondary amine. U.S. Pat. No. 4,022,755 describes cyanato-group containing phenol resins, and a process for preparing same. U.S. Pat. No. 4,713,442 discloses a polytriazine which comprises 1, 3, 5-triaryloxytriazines. Polyaromatic cyanates are also disclosed in EPA 0 147 548, WO85/03713 and GB-A-1218447.
Cyanato group containing phenolic resins have been described in Delano, et al, Synthesis of Improved Phenolic Resins, Acurex Corp/Aerotherm, Acurex Vinyl Report 79-25/AS, Sep. 4, 1979 prepared for NASA Lewis Research Center, Contract No. Nas3-21368, and is available through the United States Department of Commerce National Technical Information Service.
A recent reference, Heat Resistance Polymers by Critchley, et al, pp. 406-408 Plenum Press, New York, 1986 has described phenolic triazine resins prepared from phenolic novolac or meta-cresol novolac which have essentially the same chemical structures as described in the above referenced patents.
The phenolic triazines which have been disclosed have been found to have high thermal stability. However they have not been commercially produced because of poor shelf life, and a gel time too short for processing using conventional plastic processing equipment. It has been found as illustrated below, that reproduction of the phenolic cyanate ester resins disclosed in the art are unstable and not suitable for commercial applications such as matrix for various composites, impregnation media for paper and nonwovens, adhesives, coatings, etc. When these unstable resins are converted into a crosslinked product (phenolic triazines) mechanical properties have been observed to be poor. The cured resins are so brittle, that frequently a suitable test sample for property determination cannot be fabricated. It has been found that curing the phenolic cyanate ester resins prepared according to the disclosures in the art, generates smoke and volatile chemicals.
Various new polymers have been proposed. For example, Kunstoffe, Bd, 58, pp. 827-832 (1968) by R. Kubens, et al. and Dokl, and Akad, Nauk SSR Vol. 202, pp. 347-350 (1972) by V. V. Kovshak, et al. describe the "cyclotrimerization" of aryl cyanurate and properties of crosslinked polymers derived therefrom. The term "cyclotrimerization" is meant, forming a cyanurate ring system by chain extension polymerization of three aromatic cyanurate groups to form a crosslinked triazine ring system. Similarly, U.S. Pat. Nos. 3,890,272; 4,118,377; and 3,929,713 describe the formation of poly (bismaleimides) and the properties of such polymers.
U.S. Pat. No. 4,157,360 describes thermoformable compositions comprising a crosslinked polycyanurate polymer and a thermoplastic polymer in which the poly cyanurate is formed by a polycyclotrimerization reaction.
Phenolic cyanate resins have, according to their structural potential to form crosslinking products with outstanding thermal, oxidative stability as well as very high char yield upon heating to very high temperatures (900.degree.-1,000.degree. C.). However, resins prepared according to the teachings in the art have failed to yield products with these mechanical and thermal properties. Phenolic cyanato polymers (resins) have been found to give off smoke from volatiles upon curing. The volatiles include harmful irritants such as diethyl cyanamide. The present invention overcomes the shortcomings that have held back this potentially valuable resin from widespread commercial use.